Accurate free chlorine determinations are important to many industries and water types. It is a required reporting parameter for many regulating agencies such as EPA and FDA. Free chlorine is considered the most effective form of chlorine disinfection in applications such as drinking water production, reuse water applications, food and poultry processing operations, and in general water use where microbial protection is required. The over-estimation of free chlorine concentrations impacts the level of actual disinfection capacity available. It is well documented in analytical methods approved for reporting free chlorine concentrations that false high concentration levels of free chlorine may be obtained when interfering substances are present.
Traditionally, treated domestic wastewater is disinfected by the addition of chlorine. More recently, many drinking water facilities have converted to chloramination to disinfect potable water. Chlorine reacts quickly with ammonia (present or added) and any organic nitrogen present in the water to form monochloramine, dichloramine (from ammonia) and organic chloramines (from organic nitrogen compounds). The relative amounts of mono-, di- and organic chloramines formed during the chloramination process depend on the ratio of chlorine-to-nitrogen, pH, temperature, mixing efficiency, and time of contact. Monochloramine and dichloramine (inorganic chloramines) are very effective biocides, but organic chloramines, as a class, have poor disinfection properties.
Monochloramine is the preferred disinfectant for most wastewater treatment facilities that employ biological-oxidation treatment processes (known as secondary treatment). Prior to disinfection, most secondary treatment plants will contain ammonia levels to between 0.5 and 10 mg/L (as nitrogen, N). At pH values between 7 and 8, and when the mass ratio of chlorine to ammonia-nitrogen is 5:1 or less, all chlorine added is converted to monochloramine. When the applied chlorine (as Cl2) to ammonia-N ratio exceeds 5:1 by mass, dichloramine is formed with a corresponding drop in the total biocide concentration (monochloramine+dichloramine, expressed as Cl2). Adding additional chlorine to the water eventually consumes all of the ammonia present, and a free chlorine residual emerges usually beyond a Cl2:N ratio of about 9:1 by mass. This phenomenon is known as breakpoint chlorination and is depicted in FIG. 1.
Although a superior disinfectant, dichloramine formation is usually avoided since chlorine is unnecessarily consumed which results in a corresponding decrease in total oxidant concentration. Also, the presence of dichloramine can lead to pungent odors in the chlorine contact chambers of some secondary treatment facilities. Dichloramine is not desirable in potable water since its presence can affect both taste and odor.
According to White, “Handbook of Chlorination”, Van Nostrand/Reinhold, 3rd Ed., New York, pp. 589-606 (1993), secondary biological wastewater treatment can produce soluble organic nitrogen concentrations in the range of 3-15 mg/L (as N). It is also stated that if the mixing of chlorine (either gaseous or liquid soda bleach) with the wastewater is poor, the chlorinated species will tend to divide between monochloramine and organic chloramines. Several studies have shown that organic chloramines have significantly less germicidal activity than monochloramine.
Studies by Yoon and Jensen, Water Environ. Res. 67, 842 (1995) and Isaac and Morris, Environ. Sci. Technol. 17, 739 (1983), have indicated that, with time, monochloramine can transfer its chlorine to nitrogenous organics, producing weaker disinfecting organic chloramines. Thus, the germicidal efficiency of chlorinated wastewater has a tendency to decrease with time.
Adequacy of disinfection may be achieved by maintaining a total oxidant residual. One way to control chlorination is by monitoring the total chlorine residual, known as Chlorine Control by Residual (CCR). In the CCR process, analytical measurements are made either manually (for example, laboratory or field testing) or automatically (for example, a process analyzer). All of the commonly used methods of analyses for CCR are based on classical iodometric chemistry. Iodide, added as a reagent, is oxidized by monochloramine, dichloramine and most organic chloramines to the tri-iodide ion:NH2Cl+3I−+H2O+H+→NH4OH+Cl−+I3−.NHCl2+3I−+H2O+H+→NH4OH+2Cl−+I3−.OrgNH—Cl+3I−+H+→OrgNH2+Cl−+I3−.
In the foregoing reactions, NH2Cl represents monochloramine, I3− represents tri-iodide ion, NHCl2 represents dichloramine, and OrgNH—Cl represents organic chloramines. The resulting tri-iodide, which is formed in direct proportion to the amount of oxidant present, is measured in several ways:
1. Colorimetrically
An indicator, such as N,N diethyl-p-phenylenediamine (DPD) is added and the tri-iodide oxidizes the indicator to a colored form, which can be measured by visual comparison, or suitable instrumentation (e.g., photometer, colorimeter or spectrophotometer). A variation of this procedure is colorimetric titration, in which after reaction of the tri-iodide with DPD, the colored product is titrated against a redox titrant, such as ferrous ammonium sulfate, to a colorless end-point.
2. Amperometrically
The tri-iodide ion may be measured using an amperometric system, consisting of a probe or cell containing dual platinum electrodes or two dissimilar electrodes (e.g., silver/platinum) and a voltage generator. A small voltage is applied across the electrodes and the resulting current is compared to a standard reference potential. A variation of this technique is amperometric titration in which the generated tri-iodide is reacted with a standard reducing titrant, such as phenylarsine oxide or sodium thiosulfate. The current decreases with decreasing concentration of tri-iodide until no tri-iodide remains, the end-point being signaled when the current no longer changes. Another variation is known as the back-titration method, in which the released tri-iodide is reacted with a known excess amount of a reductant, such as phenylarsine oxide or sodium thiosulfate. The remaining reductant is titrated with standard iodate-iodide reagent, the end-point being determined amperometrically or visually using the starch-iodide end-point.
3. Direct Titration with Visual Indication
The generated tri-iodide is titrated against standard thiosulfate titrant to a visual starch-iodide end-point.
The iodometric methods currently used for CCR are not specific for the preferred disinfectant, monochloramine. The CCR-iodometric process may overestimate the disinfection efficiency due to the presence of the poorer-disinfecting organic chloramines. Organic chloramines will be present in chlorinated wastewater due to poor mixing, chlorine transfer, or nitrification (which is explained below). Organic chloramines interfere with all of the common methods used for CCR.
Under certain circumstances, nitrification may occur in secondary-treated wastewater, where the ammonia in the wastewater is partially oxidized to nitrite. With low ammonia levels, chlorination of nitrified waters may result in direct chlorination of any organic amines present, thereby decreasing the monochloramine disinfectant level in the chlorinated water and increasing the organic chloramine level therein. Conventional CCR processes may indicate an adequate disinfection level, when, in fact, disinfection efficiency has diminished.
A second process of controlling chlorination is by use of Oxidation-Reduction Potential (ORP). ORP is based on the concept that it is the oxidative potential derived from the residual that kills the microorganisms and not the concentration of the residual. Instead of maintaining a residual, ORP chlorination control maintains a certain ORP value, measured in millivolts. FIG. 2 shows typical ORP values for different concentrations of monochloramine, dichloramine and a mixture of three organic chloramines. The organic chloramine mixture includes N-chloro-butylamine, N-chloro-diethylamine and a chlorinated tri-peptide of alanine, which is representative of organic chloramines found in chlorinated wastewater effluents.
As shown in FIG. 2, ORP can be used to distinguish between pure solutions of dichloramine and monochloramine, but cannot distinguish between monochloramine and any organic chloramines present. Therefore, the weaker disinfecting organic chloramines will also affect ORP chlorination control.
Some wastewater facilities using chlorination have difficulty meeting microbial limitations although residual testing (CCR) indicates the disinfectant concentration should be sufficient. Likewise, facilities that depend on ORP for chlorination control may experience difficulty in meeting effluent limits for disinfection, although ORP values indicate sufficient oxidation potential.
Common contaminants, including iron, manganese, hydrogen sulfide, nitrate, nitrite, ammonia, monochloramine, dichloramine, organic nitrogen, and total organic carbon, have been reported to consume free chlorine and produce false free chlorine readings based on conventional measurements. See Spon, Opflow, (June 2008) pp. 24-27. In one example, a water sample required 2.323 mg/L to satisfy the total chlorine demand. That amount is significant, considering a mandated maximum residual disinfectant level of 4.0 mg/L in public drinking water. See id. at 27. The total chlorine demand and the maximum residual disinfectant level generate a narrow window for disinfecting water, and inaccurate free chlorine readings complicate such efforts.
U.S. Pat. No. 6,315,950 B1 to Harp et al. discloses methods for disinfecting water employing monochloramine. The concentration of monochloramine is measured by reacting the monochloramine with a phenol or naphthol to form an indophenol or indonaphthol that can be detected. The '950 patent is incorporated herein by reference in its entirety.